1. Field of the Invention
The invention relates in general to tap changer systems, and more specifically to load tap changer systems having protective apparatus for preventing a tap change in the event of a malfunction.
2. Description of the Prior Art
Tap changer apparatus for changing taps on a transformer winding without disconnecting the load, insolate a branch circuit of the tap changer, which branch includes a winding of a preventive autotransformer or divided reactor and the selector arm which will make the tap change. the interruption of this branch circuit is accomplished by a load switch, such as a vacuum switch, without interrupting current flow to the load through another branch circuit of the tape changer system. If the tap changer apparatus malfunctions during the tap change cycle and does not operate to isolate the branch circuit which includes the selector arm to be moved, the selector arm would be damaged if allowed to move from one contact to another. Thus, protective circuits are provided which monitor a predetermined parameter of the tap changer system and prevent a tap change in the event the monitored parameter indicates a malfunction during a tap change cycle. U.S. Pat. No. 3,622,867, which is assigned to the same assignee as the present application, discloses a load tap changer system with protective apparatus which monitors current flow at a selected point and at a selected time during a tap change cycle. In one embodiment, operation of the vacuum switch is monitored with a single current transformer just prior to movement of a selector arm, to determine if current is flowing through the vacuum switch. If current is flowing at this time, the vacuum switch failed to interrupt current flow and thus current is still flowing through the selector arm. An output of the current transformer at this time operates a protective or lock-out relay which has contacts in the tap changer drive circuit. In another embodiment, the bypass switch and the vacuum switch are both checked for proper operation by monitoring current flow through the selector are to be moved, just prior to movement thereof. If current is flowing through the selector arm, the bypass switch has failed to open the branch circuit, or the vacuum switch has failed to interrupt current flow between the two branch circuits. Two current transformers are required for this embodiment, with a single lock-out relay being responsive to the output of either. An output by a current tranformer when it is being checked just prior to movement of a selector arm, is used to operate a protective relay which has contacts in the tap changer drive circuit.
The protective or lock-out relay responsive to output current from a monitoring current transformer has a predetermined operating current range. Certain load tap changer applications operate over a tap range which may cause an exicting current through the split reactor or preventive autotransformer when operating on certain taps which will cause the secondary current of the monitoring current transformer to be outside the operating range of the protective relay.